Fluids in nanopores with diameters of <100nm exhibit behavior that is not seen at micrometer dimensions and above, such as ion current rectification, ionic selectivity, size exclusion and potential dependent ion concentrations in and near the pore. These properties originate from electrostatic interactions between charges on the nanopore surface and the fluid within the nanopore. The influence of these electrostatic effects is determined from a characteristic screening length for the system known as the Debye length. Typical nanopore systems have diameters on the scale of the Debye length and require the consideration of electrostatic effects which do not need to be considered in micrometer systems. Nanofluidic components may be designed by considering the effect of these surface interactions to control ionic transport and incorporate them in devices.In this study we present single conically shaped polymer nanopores with controlled chemistry of the pore walls and pore opening diameter between 5 nm and 30 nm. Two types of pores were examined. The first group of pores contained a junction between two zones with different surface charges. The first group consists of bipolar diodes, which have a two zones composed of positive and negative surface charges, and unipolar diodes, which have two zones composed of a charged zone and a neutral zone. We find that both bipolar and unipolar diodes show a substantial increase in asymmetrical behavior of current-voltage curves over a conical nanopore with a uniform surface charge. Further is it shown that while both diodes show an increase in current rectification, bipolar diodes in particular have superior rectification abilities. The second group of pores are modified by tethering single-stranded DNA molecules to the pore wall. We find that the DNA occludes the narrow opening of nanopores and that the this occlusion effect decreases with an increase in the concentration of the electrolyte. The results are explained by the persistence length of DNA. At low KCl concentrations (10 mM) the molecules are in an extended configuration, thereby blocking the opening and restricting the flow of ionic current to a greater extent than for high salt concentrations. Attaching DNA creates a system with varying opening diameters that can be used to control neutral and charged species.
|Advisor:||Siwy, Zuzanna S.|
|Commitee:||Dennin, Michael, Martens, Craig|
|School:||University of California, Irvine|
|Department:||Physics - Ph.D.|
|School Location:||United States -- California|
|Source:||DAI-B 74/10(E), Dissertation Abstracts International|
|Keywords:||Bipolar diodes, Debye length, Electrostatic interactions, Nanopores, Voltage-gated channels|
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